Many control devices may be pneumatic based. Pneumatic based control devices may control various systems based on a gas flow or pressure. Typically, such pneumatic control devices may include a “flapper” technology that may regulate a gas flow to thereby provide a pneumatic control signal.
One example of a pneumatic control device is a pneumatic thermostat. Pneumatic thermostats may be used as sensing and control devices for pneumatically controlled devices, such as variable air volume (VAV) units, ventilators, fan coil units, reheat coils, radiators, and the like, typically employed in a heating, ventilation, air conditioning (HVAC) system.
One type of pneumatic thermostat includes a pneumatic temperature controller, a setpoint cam, and a knob/slider. Such a pneumatic temperature controller may be a combination of a valve unit (typically a diaphragm type valve), a “flapper” controlled nozzle, and a bimetallic strip. A supply air is passed through the valve unit which controls the pressure at an outlet, after allowing a portion of the supply air to exit into the atmosphere through the flapper controlled nozzle. The outlet pressure can be used to pneumatically control another device.
Changes in a position of a flapper over the control nozzle may create corresponding changes in the amount of supply air exited to the atmosphere. This, in turn, may change the outlet air pressure.
A setpoint for such a pneumatic temperature controller may be manually set, by adjusting a cam position using a knob or slider. A cam position may change the amount of force applied by the bimetallic strip to the flapper. The position of the flapper may thus be determined by a resulting balance between by the force exerted from the portion of supply air passing through the nozzle on one side, and the force generated by the bimetallic strip on another side. A force generated by a bimetallic strip may be proportional to a difference between a setpoint and the ambient temperature for the pneumatic thermostat.
In the above arrangement, when the ambient temperature is at the setpoint, the flapper may reach an equilibrium position, creating a certain clearance above the nozzle, which in turn creates a corresponding outlet pressure. However, when the ambient temperature is away from the setpoint in one direction, the bimetallic strip exerts less force on the flapper. This may move the flapper away from the nozzle increasing a clearance between the flapper and nozzle. Such increased clearance may allow more supply air to escape to the atmosphere, reducing the outlet pressure. Conversely, when the ambient temperature is away from the setpoint in the other direction, the bimetallic strip exerts greater force on the flapper. This may move the flapper closer to the nozzle, decreasing a clearance between the flapper and nozzle. Such decreased clearance results in less supply air escaping to the atmosphere, increasing the outlet pressure.